It is of utmost importance to be able to control all of the steps used in the processing of ceramic bodies in order to obtain a reliable product. In addition, it should be possible to achieve chemical and microstructural homogeneity in complex shapes thus produced. Further, when several different ingoing components, such as submicron powders, whiskers, platelets, etc., are incorporated in advanced ceramic materials, good homogeneous mixing of such ingredients is a necessity. Such components have a cohesive nature in the dry state. Consequently, good mixing at the particle level is difficult to achieve. In general, the components are mixed with a liquid, a proper dispersant and possibly further additives so that a well dispersed, non-agglomerated ceramic slurry can be made. In this way, it is possible to obtain a very good homogeneous powder mixture. After mixing, a solid-like body often referred to as a green body with a well defined shape is manufactured and ideally, the almost perfect distribution in the slurry is preserved. In addition, the microstructure of the green body, e.g., spatial density variations, pore size distribution and phase homogeneity, have to be carefully controlled.
Present methods of forming ceramic green bodies are deficient in that they generally degrade the chemical and/or microstructural homogeneity of the mass of particles in the suspension which results in more or less decreased material properties. For example, dry pressing after spray-drying of the slurry, often leads to density variations due to non-uniform compaction and a retained agglomerate structure from the spray-dried granules. Further, with all types of drained casting techniques, such as slip casting, pressure casting, pressure filtration and centrifugal casting, the phase homogeneity and the orientation of non-spherical constituents such as whiskers are affected by the liquid flow. In particular, mass-segregation may occur due to differences in particle size and density. Also, whiskers are generally oriented perpendicularly to the flow direction.
Undrained forming methods such as injection molding have the potential to avoid the foregoing problems. However, due to the high viscosity of the polymer matrix, care has to be taken to achieve good mixing. It is also important to ensure proper burn-out of the polymer matrix. Further, the mold has to be designed in a way which takes into account the whisker orientation which often occurs due to high stress fields during the molding process.
Recently, some gel-forming methods have been disclosed. For example, U.S. Pat. No. 4,894,194 ("Janney"), the disclosure of which is hereby incorporated by reference, discloses a method of molding ceramic powders by mixing the ceramic powder with a dispersant and a monomer solution. The mixture is transferred to a mold and the monomer is crosslinked to form a polymer network giving sufficient strength to the ceramic green body. In addition, EP-A-0246438 ("Fanelli") discloses a method of injection molding an aqueous mixture of a gel-forming material, a dispersant and a ceramic powder.
U.S. Pat. No. 4,541,855 ("Scherer"), the disclosure of which is hereby incorporated by reference, discloses a method of forming a glass or ceramic product by direct casting of a non-aqueous sterically stabilized suspension of oxide parities. The method includes adding a chemical agent to the stable suspension to cause delayed gelation presumably by displacing the dispersant after some time. Before the suspension gels, it is transferred to a mold of desired shape.
However, the above gel-forming techniques also have drawbacks. For instance, the gel-forming reaction is more or less irreversible. That means that a green body which does not fulfill any of the required specifications such as shape tolerances or packing density has to be rejected and cannot be remolded. Also, as these forming methods consist of several different components which might interact with each other in an unexpected way, this can lead to aggregation or unwanted suspension properties.